Geochemical and Reservoir Characteristics of the Middle-High-Mature Shale of the Jurassic Lianggaoshan Formation in Northeast Sichuan Basin and Their Influence on Shale Oil Enrichment

Abstract

The Lianggaoshan Formation exhibits significant potential for shale oil and gas exploration, representing a crucial area for discovering substantial petroleum reserves in the Sichuan super gas-rich basin. However, due to its complex geochemical characteristics and pore structures, the primary controlling factors of shale oil enrichment remain insufficient, hindering the evaluation of favorable zones for shale oil enrichment in the Lianggaoshan Formation. This study examines the geochemical, petrological, and reservoir characteristics of the Lianggaoshan Formation to elucidate the shale oil enrichment mechanisms. The Lianggaoshan Formation shale can be classified into seven main lithofacies types: organic-rich laminated siliceous shale (ORLSS), organic-medium laminated siliceous shale (OMLSS), organic-lean bedded siliceous shale (OLBSS), medium to organic-rich laminated clay shale (M-ORLCS), medium to organic-rich laminated mixed shale (M-ORLMS), medium to organic-lean massive (fine) siltstone (M-OLMFS), and medium to organic-lean massive-limestone (M-OLML). In terms of source rock quality, the Lianggaoshan Formation shale exhibits high organic matter abundance (TOC = 0.2-4.2%), type I and II kerogen, and mid-to-high-maturity stage (R0 = 1.48-1.66%), indicating strong hydrocarbon generation potential and providing a robust foundation for shale oil enrichment. Regarding reservoir conditions, the Lianggaoshan shale is primarily composed of quartz and clay minerals with diverse pore types, including intergranular, intercrystalline, and intragranular dissolution pores and organic matter pores. Additionally, microfractures, such as structural fractures, interlayer fractures, bedding fractures, and intercrystalline fractures, are well-developed. Among the lithofacies, ORLSS, OMLSS, and M-ORLCS exhibit best physical properties, with ORLSS and OMLSS featuring well-developed macropores and bedding fractures, while M-ORLCS, M-ORLMS, and ORLSS have the highest S1 values and OSI, indicating favorable oil-bearing properties. The organic-rich laminated shale demonstrates a strong source-reservoir integration relationship, where high-frequency lamination, high TOC and free oil content, abundant pores and network fractures, and strong connectivity facilitate micromigration and in situ accumulation of generated oil. This study enhances the understanding of medium-to-high-maturity lacustrine shale oil enrichment, providing a scientific basis for predicting and exploring shale oil “sweet spots” in the Lianggaoshan Formation while offering valuable insights for optimizing exploration strategies in similar basins.